Digital-to-analog converters are common components in semiconductor integrated circuits. In general, a D2A converter converts a given digital value to an analog signal (e.g., an analog voltage) associated with the given digital value. Currently, D2A converters can be designed to provide various resolutions. However, the higher the resolution of a D2A converter, the greater the die area occupied by the D2A converter is. Thus, it is more difficult to design a D2A converter with a high resolution when the die area is limited.
Most D2A converters today typically have a 1-stage topology. For example, a typical existing D2A converter includes a long resistor ladder. The resistor ladder is driven from the top by some higher voltage (Vtop) and at the bottom by some lower voltage (Vbot, such as ground). There are tap points along the resistor ladder that provide a range of voltages in between Vtop and Vbot. The voltage step between two adjacent tap points is referred to as the resolution of the D2A converter. Today, a typical D2A converter used in some flash memory devices generally has a resolution of about 50 mV.
In order to build a D2A converter with a higher resolution, say 10 mV, using the same type of 1-stage topology, the tap points are placed much closer together. When the tap points are closer together, the circuit components (e.g., switches, etc.) that are fitted into the space from one tap point to the next are crowded together. The space from one tap point to the next is also referred to as a “tap pitch.” In some integrated circuits, the tap pitch may be limited by other factors, such as contact-to-contact spacing or other litho constraints. Hence, the crowding of the circuit components worsens in these integrated circuits.
To alleviate the above problem, the tap points are pulled further apart in some existing D2A converters, but this causes the size of the overall D2A converters to increase. For example, to increase the resolution of an existing D2A converter from about 50 mV to about 10 mV, the circuit length (and thus, the circuit area) may have to increase by a factor of 5. This growth is due to both the additional resistors in the resistor ladder itself and the additional switches used to select tap points because 5 times as many switches may be used to increase the resolution by a factor of 5. Furthermore, to maintain the same resistance per tap point, the resistor ladder has to be made wider while keeping the length-to-width ratio of the resistor ladder substantially the same. Hence, the overall circuit area may increase by more than a factor of 5. In some instances, the resistor ladder area increases by a factor of as much as 25.